Vol. 57.] UPPER GREENSAND AND CHLOKITIC MABL OF WILTSHIRE. 121 



larger region. The point of these remarks will be seen in the 

 sequel. 



Let us first take the local succession which has been described, 

 and endeavour to determine what horizon is suggested by the 

 evidence as the most convenient plane of division between the 

 Selbornian and the Cenomanian in this part of Wiltshire. 



At Maiden Bradley the beds through which a passage takes place 

 show no decided break of continuity, though they are appreciably 

 distinct one from the other, and the upper surface of the Cornstone- 

 Bed is a fairly well-marked plane. StiU no one could decide, from 

 a mere inspection of the section, which was the ;nost natural plane 

 of separation between the two formations ; nor, as a matter of fact, 

 is the evidence of the fossils very decisive on the point. It might be 

 taken at the base of the Chloritic Marl, below which Stauronema 

 does not occur, and to which a few other species seem to be confined, 

 such as Hemiaster Morrisii, Pecten Beaveri, RTiynchonella Martini, 

 Salenia Austeni, and Holaster trecensis. But all the characteristic 

 ammonites of the Chloritic Marl are common in the nodule-bed 

 below, and are not unfrequent among the Cornstones ; while some 

 of them occur also at the top of the sand which rests upon the Chert- 

 Beds. 



A large number of the fossils which occur in the nodule-bed (No. 2) 

 do not occur below the Cornstone-Bed ; but, since fossils are rare in 

 the underlying sand, it may be that their absence is a mere local 

 accident. On the other hand, nearly all the fossils which have 

 been found in this sand range up into the Cornstones. 



At Rye Hill there is a still more complete sequence, and the 

 Chloritic Marl passes down into a sand which yields a large number 

 of the fossils that have been catalogued as those of the Warminster 

 Greensand. Some of the Chalk-Marl ammonites, namely. Ammonites 

 varians, A. Coupei, A. Mantelli, and A. naviculari.% occur in this 

 sand : the first abundantly, the others more rarely ; and we have 

 no hesitation in regarding this sand, with the brownish layer at 

 its base, as an expansion of the nodule-bed and brown sand of 

 Maiden Bradley. The Cornstones and the sand below them have 

 the same relations as at Maiden Bradley ; there does not appear 

 to be any kind of break, either above or below the Cornstones. 

 Fossils are so rare, in the small area of the lower sand exposed, 

 that the fauna associated with Ammonites varians does not actually 

 set in till the upper third of the Cornstone-Bed is reached. 



At Mere the base of the Chloritic Marl or Stauronema-hedi is 

 fairly well marked, and there is nothing to correspond with the 

 nodule-bed of Maiden Bradley or with the upper fossiliferous part 

 of the Eye Hill Sand. Nevertheless, there is no evidence of strong 

 erosion, and the cephalopodan fauna of the Chloritic Marl descends 

 into the Popplestone-Bed, which is the equivalent of the Corn- 

 stone-Bed of the other localities. Finally, the uneven base of the 



